Are professors biased? Is there a liberal bias against Christians on college campuses? What is the definition of liberal bias or conservative bias? The answers to these questions are complicated and subject to interpretation. So here's my personal take on things:
Yes, bias in the classroom does exist. Many professors are liberal, and therefore much of this bias is liberal. (Keep in mind, however, that some schools and professors are more liberal or conservative than others, and not all bias is liberal.). If you ask social critics such as Ann Coulter or David Horowitz, this is a very serious problem. I think they are exaggerating. Although I do think it is an extremely serious problem if professors discriminate against their students based on political or religious beliefs, I don't think that a professor's personal opinions matter as much as some people may think.
First of all, all professors have bias (as do students). Although the professor can attempt to keep the class as balanced as possible, it's impossible for that bias not to affect the class to some degree. The topics, readings, and discussions chosen by the professor will reflect how the professor sees the world.
Second, professors should have the right to express their personal opinions openly, just as students should. Wouldn't you rather know where your professor stands on the issues being discussed in class than for this to be a mystery? During the McCarthy era, professors with the "wrong" views were fired and silenced. It's important to maintain an atmosphere where everyone can express their opinions, including professors.
There's a line, however, between bias and discrimination. All professors have bias, but they have an ethical obligation to observe certain democratic practices in the classroom:
If you have a professor who discriminates against students, you should complain to the professor, the department, and/or the school. But please understand that this is a serious accusation, and that most professors who express their opinions do not discriminate against students who disagree with them. Personally, I would much rather have a student who strongly expresses opinions that are different than mine than a student who says nothing at all. Talk to the professor if you have concerns, and you may just find yourself in a friendly and intelligent conversation.